1. Field of the Invention
The present invention relates to an ignition coil which is used mainly in an internal combustion engine for a vehicle.
2. Description of the Related Art
In an ignition coil having an iron core which is provided with a primary coil and a secondary coil wound therearound to form a closed magnetic path, and a permanent magnet which is provided in a part of the iron core, high electric voltage is generated in the secondary coil by supplying a primary current to the primary coil and thereafter cutting it off, thereby to actuate an ignition plug for igniting fuel of the internal combustion engine. Although performance of the ignition coil may depend on specifications of the primary and secondary coils for this purpose, the iron core, permanent magnet, and gaps between them which form the magnetic path have been also significant factors for influencing the performance of the ignition coil. Further, the gaps usually exist at two positions, one at a side where the permanent magnet is provided and the other at an opposite side where the permanent magnet is not provided, and such accuracy as forming no clearance has been required.
[Patent Document 1] Japanese Patent No. 2857890
[Patent document 2] JP-A-6-36950
The invention of Patent Document 1 includes, as shown in FIGS. 6A to 6C, an exciting core 41 for the primary and secondary coils to be wound therearound, a closed magnetic path forming core 42, and a permanent magnet 43. There are formed gaps 44a, 44b at one position (hereinafter referred to as a first gap) in which the permanent magnet 43 is arranged, and a gap 45 at the other position (hereinafter referred to as a second gap) in which the permanent magnet is not arranged. At least one of the gaps at the two positions is so designed that contact faces of the two iron cores are inclined so as to restrain formation of the gap. Moreover, the closed magnetic path forming core 42 is covered with a covering, and the covering is provided with a projected portion for positioning the permanent magnet 43. Patent Document 2 also discloses such a structure that an exciting core having a substantially T-shape may be internally in contact with a closed magnetic path forming core having a substantially C-shape.
However, in case where the contact faces of the iron cores are inclined, it has been necessary to form the inclined faces not only in the closed magnetic path forming core 42 but also in the exciting core 41, which the primary and secondary coils are wounded so as to be consistent with the inclination of the closed magnetic path forming core 42. In this case, there has been a problem that a clearance may be formed between the contact faces, or only a part of the contact faces may get in contact, which results in disadvantages in workability and production cost. Moreover, there has been a problem that positioning of the permanent magnet has become necessary, because it has been difficult to mount the permanent magnet stably on the inclined faces, or an effective sectional area has been made small due to narrowed tip ends of the inclined iron cores, and magnetic flux density has become poor. In addition, there has been a problem that the first and second gaps cannot be decreased unless dimensional accuracy of the iron cores is controlled, also on occasion of internally contacting the exciting core with the closed magnetic path forming core in a C-shape. In view of the above described circumstances, an object of this invention is to provide such an ignition coil that the gaps can be made narrow at a lower cost.